This code follow can change a shape or picture sprite's
dimension:
vRect = mySprite.rect
vRect.left = vRect.left - 5
vRect.right = vRect.right + 5
mySprite.rect = vRect
But when this code is used to a text(or field) sprite, it
doesn't work! Why? How can I change a text box's dimension?

1. The text member's .boxType property must be set to #fixed
or #scroll
2. You need to set the .height of the member

Changing the height of the sprite or the rect of the member
or the sprite will not affect the height of the text member, so the
height of the sprite will not change. (The sprite may report having
a different height, but visually, there will be no change).